1. Field of the Invention
This invention relates to density calibration tools for setting the density output of printers, and more specifically, for using a print-out from a standard printer with known and controlled settings as a standard for comparison with output printed by a target printer sought to be calibrated.
2. Description of the Related Art
Images of varying xe2x80x9cshadesxe2x80x9d can be printed using only a black toner or ink. The xe2x80x9cshadesxe2x80x9d are produced by printing varying gray levels of black from 0 to 100% coverage. xe2x80x9cShadesxe2x80x9d or xe2x80x9cgray levelsxe2x80x9d are also referred to as half-tones. A half-tone algorithm can be used by a printer for instructing that printer as to which xe2x80x9cpelxe2x80x9d or dot of ink to print for varying coverage amounts. For example, for 100% coverage, or darkest image, the printer would print a dot in every single pel. For 50% coverage, a dot would be printed in every other pel. Certain patterns of dots are used for the various amounts of coverage. Lighter half-tones or gray levels are produced as the coverage amounts decrease.
However, another factor that affects the lightness or darkness of an image is optical density. In other words, even though a part of an image may have 100% black coverage, that part of the image may appear lighter or darker depending upon its optical density.
For example, with 100% coverage using a toner or ink that is 100% perfectly black, there would be a reflectance value of zero (0), i.e., no light would be reflected back. The formula for optical density is the log of the background divided by the reflectance of the spot that is being examined. In other words, if the reflectance had a value of 10 (which is pretty dark), but the background is white having 90% reflectance, i.e., having a value of 90, the ratio is 9. The log to the base 10 of 9 is slightly less than 1, i.e., 0.954, which would be the value of the optical density.
In order to get good images when printing, the optical density of the printed toners has to be maintained. If optical density changes in the printer, then the images will change and the image quality may not be as good as desired. In high function and high cost printers, it is known to have a densitometer built into the printer to either completely automate the control of the density, or to semi-automate density control by including operator interactions. However, for low cost printers, the cost of having a densitometer, either built into the printer or available to the operator, is prohibitive. For example, a densitometer may cost one thousand dollars, but many low cost printers sell for less than this amount. A printer manufacturer can not afford to build a densitometer into a low cost printer and still keep the printer xe2x80x9clow-costxe2x80x9d. Likewise, most users of xe2x80x9clow-costxe2x80x9d printers cannot afford the price of a densitometer as an extra tool for maintaining the optical density of the printer. Nevertheless, even for low cost printers there is a need to maintain optical density in order to provide quality print images.
In essence, monochrome halftone rendering is inconsistent among printers due to differences in the optical density produced by the printer for various gray scale levels. A densitometer is typically used to calibrate the printer to the correct optical density. However, this tool is not available to the average customer. There is a need to maintain image quality by maintaining optical density without using a densitometer.
The method, program, and tool of this invention enables an operator of a printer to set the density setting of the printer to achieve a desired image quality without using a densitometer. The density setting tool is made using a same type of printer, and same type of toner, as would be used by the operator. This same type of printer is referred to herein as the xe2x80xa2standardxe2x80xa2 printer. The standard printer is set up, such as by the printer manufacturer, with the desired density output at a given density setting. A densitometer may be used in setting up the standard printer. A separate gray scale vignette is printed for each density setting of the standard printer. The gray scale vignettes are all printed using a same half tone algorithm. The gray scale vignettes are compared with each other to determine the amount of offset, in physical distance from the center of the vignette, there is between each of the vignettes in order that a point on each vignette matches (either visually or using a densitometer) in density with a point of another vignette. The distance, or amount of offset, is then known for each density setting from the nominal setting or from each of the other density settings.
A standard grey scale vignette is printed off using the standard printer at the nominal printer setting. The standard gray scale vignette may also contain an aperture such as an aperture that is positioned at the center of the vignette. The standard vignette is made available to the operator. Along with this standard gray scale vignette is sent a file for the operator to print out using the operator""s printer. The file contains a gray scale test vignette using a same halftone algorithm as was used to print out the standard gray scale vignette. Either the standard vignette or the test vignette will contain tick marks at distances apart from each other that correspond to the distances associated with each density setting of the standard printer as determined above.
After printing out a test vignette using the operator""s printer, the operator will lay the standard vignette over the test vignette such that a portion of the test vignette can be viewed through the aperture of the standard vignette. The operator then aligns the test vignette such that the portion of the test vignette appearing through the aperture visually matches in density the portion of the standard gray scale vignette immediately surrounding the aperture. When aligned, the amount of offset between the two vignettes is noted as indicated by a tick mark corresponding to that amount of offset. The specific tick mark indicates the density setting that the operator must adjust the printer to in order to get the desired density output.